<div class="button" onclick="toggleClass()">
  <div id="signifier" class="add"></div>
</div>

html, body {
  height: 100%;
}

body {
  background: linear-gradient(#094481, #0D58A6);
}

.button {
  position: relative;
  height: 50px;
  width: 50px;
  background-color: #f50;
  border-radius: 50%;
  padding-top: 13px;
  margin: auto;
  box-sizing: border-box;
}

.add {
  position: absolute;
  margin: auto;
  height: 20px;
  width: 4px;
  background-color: #fff;
  top: 14px;
  left: 23px;
}

.add:before, .add:after {
  position: absolute;
  top: 8px;
  left: -8px;
  content: ' ';
  height: 4px;
  width: 20px;
  background-color: #fff;
}

.play {
    height: 0;
    width: 0;
    margin-left: 15px;
    border-left: 25px solid #fff;
    border-top: 13px solid transparent;
    border-bottom: 13px solid transparent;
  }

.pause {
  height: 20px;
  width: 20px;
  border: none;
  margin-left: 15px;
  border-right: 5px solid white;
  border-left: 5px solid white;
  box-sizing: border-box;
}

let listToggle = 0
const classes = ['play', 'add', 'pause']

/* if you're actually writing a music player you'd make more complicated logic than this, but from here you can see how to toggle classes with javascript */

const toggleClass = () => {
  const sig = document.getElementById('signifier')
  sig.classList = classes[listToggle++ % classes.length]
}